The present invention relates to semi-permeable membranes, and more particularly, to nanoporous membranes and methods for making and using the same for desalination and other processes.
Water desalination may be thought of in terms of two approaches. The two basic approaches for water desalination include reverse osmosis and distillation. The distillation approach requires converting fluid water to the vapor phase and condensing water from the vapor. This approach is fairly high cost and requires significant energy usage. The reverse osmosis approach uses pressure on a salinated liquid to force water molecules through a semi-permeable membrane. This approach has a relativity low rate of energy consumption.
The specific (per unit of produced potable water) energy of desalination using reverse osmosis has been reduced from over 10 kWh/m3 in the 1980s to below 4 kWh/m3, approaching the theoretical minimum required energy of 0.7 kWh/m3. To improve the state of art of the reverse osmosis approach, new membranes, with a uniform pore distribution and a more permeable separation layer can potentially maintain or improve salt rejection while increasing the flux in the reverse osmosis method. The pore size of membranes for effective desalination is on the order of 10 nanometers or less. Current processes for achieving these dimensions are rather complicated, costly, and time consuming. One method employs commercially available anodized aluminum oxide films that have parallel arrays of nanochannels having diameters of 10 to 200 nm or greater. To narrow the pore size, surface charged materials are deposited at the openings of the nanochannels by chemical vapor deposition, atomic layer deposition, or physical vapor deposition. Alternatively, an oxide layer is formed at the inlet openings by atomic layer deposition. Multiple passes are required to attain a desired thickness and pore diameter. These processes are time consuming, complex, and relatively costly to obtain the desired diameters suitable for desalination. Moreover, the current processes merely narrow the inlet opening of the nanochannel.
Accordingly there is a need in the art for improved membranes having openings less than 10 nm, wherein the process is efficient and economically practical for production purposes.